Present day installation of a new LPG (liquid petroleum gas) tank, which may range in capacity from 100 to 500 gallons and weigh between 355 and 1980 pounds, typically requires that it be towed with a trailer to the site by a truck and backed (if that is possible) into the final installation position. The towing truck must frequently have a four-wheel drive to enable its being driven over soft earth at the site. Added to the tank weight is a propane fill of approximately 10% of the tank capacity, which is accomplished during required purging noted below. Often-times, because of shrubbery, trees and plants and because of the placement in a backyard, the trailer must be unhitched well short of the final location and pushed or pulled by hand the remaining distance. If impossible to move the trailer directly to the final position, the tank must be unloaded and dragged there by hand.
When it is possible to back a truck to very near the installation point, it must be driven there by the truck and the trailer maintained hitched to the truck during unloading of the tank. For at least 60 years, the industry standard trailer has had a U-shaped frame, with the base of the U being adjacent the trailer hitch. The trailer was backed over an empty tank, the frame straddling the tank as the trailer was moved over it. A fixed-position overhead hoist then lifted the tank above the frame, cross supports were positioned below the tank and connected to legs of the U-shaped frame, and the tank was lowered to the cross supports and strapped to the frame for delivery. The empty tank was then taken or transported by the truck and trailer to a purging station where factory-installed compressed test air has to first be evacuated by purging the tank with propane vapor and alcohol. Approximately ten percent of the tank is then filled with LPG. State laws typically limit tank transportation over the road to no more than that amount in the portable tank. The vehicle and partially-filled suspended tank was then driven to the site, backed as close as possible to the point of installation and the pick-up process was reversed to lower the tank to the ground between the U frame. This often meant driving the truck and trailer across a lawn, often causing some lawn damage. The tank was manually moved the remaining distance, this oftentimes being the most physically strenuous part of the entire process. The entire procedure just described ordinarily ties up the time of two persons from initial tank pick-up to final installation. One variation of the above U-shaped trailer has a cantilevered boom extending rearwardly beyond the end of the trailer. The trailer is first backed while remaining hitched to the truck as close to the installation site as possible. If the trailer could be aligned with the final position of the tank, it could merely be lowered between the legs of the U. If, however, the tank had to be finally located at right angles to the position to which it could be driven by the truck, the tank was first lowered to the ground between the legs of the U and disconnected from the hoist, the truck and trailer driven forward until the distal end of the boom was directly over the center of the tank, and the tank then raised again, turned 90 degrees to the trailer, and lowered again to its final position. To accommodate this final move, the front hoist cable or chain would have to be removed from the front end of the tank, a separate spreader chain or cable connected between the ends of the tank, and the rear cable extended to a pulley at the end of the boom and connected to the center of the spreader chain. The tank could then be lifted again, turned 90 degrees about the rear hoist cable, and lowered to the ground. While this was done, the trailer had to remain hitched to the truck because the weight of the cantilevered tank suspended at the far end of the boom would tend to raise the hitch end of the trailer. As such, the truck, often necessarily a four-wheeled drive vehicle, had to keep the trailer hitched the entire time.
Another means of tank delivery requires two or more persons to normally mount a first pair of wheels under one end of the tank and drive wheels under the opposite end, drive the combined unit to a purging station with the tank itself serving as a chassis between the independent front and rear wheels, drive the unit onto a trailer bed by means of a ramp at the back of the trailer, tow the trailer to the installation site and park it on the road or in a driveway, drive the unit off the ramp, and then drive the unit from the road to the final resting position. To the best of my knowledge, this latter tank delivery system is the only one which enables the tank to be removed from a trailer at the road and driven to the final installation point. Nevertheless, I believe it to be necessary to employ two persons both to mount the tank onto its front and rear wheels at the outset and to remove the sets of wheels just before final installation. This essentially ties up two employees throughout the entire process, adding to the overall cost burden which includes the drive time of two people from the yard to the site and back. The manufacturer of this system also uses a special motorized cart onto which a smaller upright tank can be lifted and similarly delivered to the site. Here the tank is cargo only, and not part of the driven structure.
Use of independently-driven motorized trailers in fields other than propane tank delivery has been known for a considerable period of time, but that knowledge has not been applied to the propane industry. For example, such trailers have been used for delivering concrete burial vaults to cemeteries to avoid driving trucks over and around graves and depositing the vaults off the back end of the trailers into graves prepared in the ground. Because such vaults may weigh as much as 3000 pounds, a telescoping boom had to be used to extend well beyond the end of the trailer in order to straddle the grave. The telescoping end had a pair of legs mounted to the trailer during transporting of the vault by truck to the cemetary. The trailer was unhitched, powered by itself to one end or side of the grave, the heavy legs and boom extension detached by two persons from the trailer and manually lifted and carried to a position beyond the grave and firmly planted on secure earth at the far end of the grave. Whatever the reason, this readily-available, long-used design was never used in the propane tank industry, possibly because it was manual labor intensive in extending and returning the legs and perhaps also because it was unsuitable without extensive modification and redesign in order to accommodate it to the needs of the LPG industry.